Flat gaskets, in particular for a sump employed in internal combustion engines, must be capable both, of taking clamping loads and that of ensuring fluid-tightness.
For this purpose, the existing flat gaskets generally comprise a sheet of metal which permits taking loads and has a profile cut out to the shape of the joint plane.
The sheet is also provided with openings corresponding to the passage of clamping bolts and openings for the passage of the fluids.
On the periphery of the openings corresponding to the passage of fluids there is provided a beading of an elastomeric material which provides a seal.
A first important drawback of prior art gaskets is the poor connection of the elastomeric beading to the metallic sheet.
Indeed, the beading is molded and then secured by bonding solely on the inner surface of the opening. Since, the sheets employed have a thickness of a few tenths of a millimeter this surface is very small and, notwithstanding the fact that the bonding is considerable, the mechanical connection remains too weak to resist certain mechanical stresses, vibrations or shocks.
Moreover such gaskets provide a primary seal solely by means of the elastomeric beading.
There is no secondary seal which would complete this primary seal, so that any defect in the elastomeric beading results in leakage.
In an attempt to overcome this drawback, prior art gaskets have been provided in each of the openings with a projecting annular region which permits increasing the bonding area.
In certain cases, improvements involve radially notching the gaskets to still further increase the bonding area.
Nonetheless, such mechanical connection have been found insufficient for many applications since they provide no secondary seal and do not permit varying the distribution of the clamping loads.
Some gaskets of the prior art propose a stack of sheets coated with a thin layer of elastomer.
On the periphery of the openings to be sealed, the sheets are deformed so as to provide a boss.
In this case, the primary seal is ensured by the boss and the thin layer of elastomer while the secondary seal is ensured by the layer of elastomer spread over the joint plane.
For gaskets of this kind, the drawback concerns the elastomer, since it is selected from the nitriles family and thickness limited to be a few tenths of a millimeter and since a material is liable to crack at greater thicknesses.
Such a gasket thus only has a limited capacity to receive imprints since the elastomer does not have a good creep quality.
Consequently, the joint planes must have a very carefully prepared surface state, which correspondingly increases the cost and samplexity of manufacture.
There is also known from FR-A-1 333 891 a gasket which comprises a metallic core covered with a sealing material and which comprises extra thicknesses in the region of the openings to be sealed. This gasket has no means adapted to take loads well.